Photoacoustic measurement device and laser light source

ABSTRACT

A flash lamp  32  excites a laser rod  31.  A Q switch  35  which changes the loss of the optical resonator according to the voltage applied is inserted on the optical path of a pair of mirrors  33  and  34  forming the optical resonator. An optical path shutter  39  is provided on the optical path of laser emission light. In a first operation mode in which laser emission is performed, the optical path shutter  39  is opened and the voltage applied to the Q switch  35  is changed from a high voltage to, for example, 0 V to emit pulsed laser light after the flash lamp  32  excites the laser rod  31.  In a second operation mode in which the laser emission is interrupted and waited for, the optical path shutter  39  is closed and the voltage applied to the Q switch  35  is, for example, 0 V.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT International Application No.PCT/JP2014/057064 filed on Mar. 17, 2014, which claims priority under 35U.S.C §119 (a) to Japanese Patent Application No. 2013-061502 filed onMar. 25, 2013. Each of the above application(s) is hereby expresslyincorporated by reference, in its entirety, into the presentapplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photoacoustic measurement device, andmore particularly, to a photoacoustic measurement device that detectsphotoacoustic waves generated in a subject after light is emitted to thesubject. In addition, the invention relates to a laser light source usedin the photoacoustic measurement device.

2. Description of the Related Art

An ultrasonic inspection method has been known as a kind of an imageinspection method which can non-invasively inspect the inside of aliving body. In ultrasonic inspection, a probe which can transmit andreceive ultrasonic waves is used. When ultrasonic waves are transmittedfrom the probe to the subject (living body), the ultrasonic waves arepropagated through the living body and are reflected from a tissueinterface. The probe receives the reflected sound waves and a distanceis calculated on the basis of the time required for the reflectedultrasonic waves to return to the probe. In this way, it is possible toimage the internal structure of the living body.

In addition, photoacoustic imaging has been known which images theinside of a living body using a photoacoustic effect. In general, inthis photoacoustic imaging, the living body is irradiated with pulsedlaser light. In the inside of the living body, the tissue of the livingbody absorbs the energy of the pulsed laser light and ultrasonic waves(photoacoustic signal) are generated by adiabatic expansion caused bythe energy. The photoacoustic signal is detected by, for example, aprobe and a photoacoustic image is formed on the basis of the detectedsignal. It is possible to visualize the inside of the living body on thebasis of the photoacoustic signal.

In the photoacoustic imaging, in many cases, a Q switch laser is used asa light source for emitting pulsed laser light. In the Q switch laser, aQ switch for controlling optical loss is provided in an opticalresonator. For example, a Pockels cell is used as the Q switch. Until alaser medium is sufficiently excited, the Q switch is turned off toincrease the loss of the optical resonator, thereby suppressingoscillation. When the Q switch is switched from the off state to an onstate after the laser medium is sufficiently excited, a sufficientamount of stored energy oscillates at one time and a giant pulse whichhas very high intensity and a short pulse width is obtained. The Qswitch returns to the off state after the pulsed laser light is emitted.

For example, JP2011-224205A discloses a photoacoustic image generationdevice using a Q switch laser. In JP2011-224205A, plates for pressing asubject, a detector for detecting photoacoustic waves, and a Q switchlaser serving as a light source are provided in an exterior cover forshielding light. A door for manual operation is provided in the exteriorcover. An operator, such as a doctor, puts a hand through the door formanual operation and interposes the subject between the plates. Afterthe subject is interposed between the plates, light is radiated to thesubject and photoacoustic waves generated in the subject are detected.

In addition, JP2011-224205A discloses a structure in which, when thedoor for manual operation is opened, it is determined whether to stopthe emission of light from the Q switch laser or to shield laser light.When a door opening and closing detection sensor detects that the doorfor manual operation is opened, a control unit turns off the Q switch ofthe Q switch laser. In addition, the control unit closes a shutter whichis provided on the optical path of the laser light such that the laserlight emitted from the Q switch laser is not radiated to the subject.JP2011-224205A also discloses that it is preferable to turn off the Qswitch and to close the shutter in order to further improve safety.

SUMMARY OF THE INVENTION

Here, for example, it is considered that the photoacoustic imagegeneration device is generally used in the following situations: thenumber of times the photoacoustic image is continuously observed for along time is small; and the photoacoustic image is observed in a shorttime before and after a surgical operation. When the photoacoustic imagegeneration device is in a standby mode without generating thephotoacoustic image, it is not necessary to emit laser light from thelaser light source. However, when the excitation is stopped, thetemperature of a laser rod is likely to be changed, which results in achange in light emission conditions. Therefore, it is preferable that,even in the standby mode, for example, the flash lamp is continuouslyturned on to periodically excite the laser medium. Since the excitationis not stopped, it is possible to constantly maintain the temperature ofthe laser medium. In this case, when the Q switch is maintained in anoff state, it is possible to prevent laser light from being output tothe outside.

However, the usage patterns of the Q switch include a pattern (firstpattern) in which the Q switch is turned on to function as aquarter-wave plate and is turned off to transmit light, without changinga polarized state, and a pattern (second form) in which the Q switch isturned off to function as the quarter-wave plate and is turned on totransmit light, without changing a polarized state. In the firstpattern, no voltage is applied to the Q switch until the excitation iscompleted and a high-voltage pulse of, for example, about 3 kV isapplied to the Q switch at the emission time of pulsed laser light. Thelevel of the applied voltage is determined by the material forming thePockels cell used and the wavelength. In the second pattern, a highvoltage of about 3 kV is continuously applied to the Q switch until theexcitation is completed and the voltage applied to the Q switch isreduced to 0 V at the emission time of the pulsed laser light (negativepulse).

In the first pattern, when no voltage is applied to the Q switch, the Qswitch is turned off. Therefore, when the device waits for lightemission while continuously performing excitation, it is not necessaryto continuously apply a high voltage to the Q switch. However, when theQ switch transmits light without changing the polarized state, it isnecessary to provide a separate quarter-wave plate in the opticalresonator in order to increase the optical loss of the opticalresonator. In contrast, in the second pattern, it is not necessary toprovide a quarter-wave plate in the optical resonator and it is possibleto simplify the structure of the optical resonator and thus reduce thesize or costs of the device. However, in the second pattern, when onlyexcitation is performed with the Q switch turned off, it is necessary tocontinuously apply a high voltage to the Q switch. When a high voltageis continuously applied to the Q switch, the Q switch deteriorates.

In JP2011-224205A, when the door for manual operation is opened, the Qswitch is turned off in order to stop the emission of laser light. Inthe second pattern, when the Q switch is used, it is possible tocontinuously apply a high voltage to the Q switch. JP2011-224205A alsodiscloses a structure in which the shutter is closed to shield laserlight. In this case, the Q switch is controlled by the same method asthat used for general laser emission. Therefore, a high voltage needs tobe continuously applied to the Q switch except for during a short timefor which the Q switch is turned on. Therefore, JP2011-224205A does notdisclose means for solving the above-mentioned problems.

The invention has been made in view of the above-mentioned problems andan object of the invention is to provide a photoacoustic measurementdevice and a laser light source which can simplify the structure of anoptical resonator and suppress deterioration of a Q switch even when awaiting time for which no light is emitted is long.

In order to achieve the object, according to the invention, there isprovided a photoacoustic measurement device including: a laser lightsource; an acoustic wave detection unit that, after light is emittedfrom the laser light source to a subject, detects a photoacoustic wavegenerated by the emission of the light; and a photoacoustic signalprocessing unit that performs signal processing for the photoacousticwave. The laser light source includes: a laser medium; an excitationunit that excites the laser medium; an optical resonator including apair of mirrors that face each other with the laser medium interposedtherebetween; a Q switch that is provided on an optical path of theoptical resonator and changes optical loss of the optical resonator,according to a voltage applied, such that the optical loss of theoptical resonator when a first voltage is applied to the Q switch ismore than the optical loss of the optical resonator when a secondvoltage lower than the first voltage is applied to the Q switch; and anoptical path shutter that is provided on an optical path of laseremission light and switches between the transmission and blocking of thelaser emission light. In a first operation mode in which laser emissionis performed, the optical path shutter transmits the light from thelaser light source and the voltage applied to the Q switch is changedfrom the first voltage to the second voltage to emit pulsed laser lightafter the excitation unit excites the laser medium. In a secondoperation mode in which the laser emission is interrupted and waitedfor, the optical path shutter blocks the light from the laser lightsource and the second voltage is applied to the Q switch.

In the photoacoustic measurement device according to the invention, inthe second operation mode, the excitation unit may periodically excitethe laser medium.

The second voltage may be 0 V.

The Q switch may cause a predetermined phase difference between apolarized component which is parallel to an optical axis of transmittedlight and a polarized component which is perpendicular to the opticalaxis when the first voltage is applied and may not cause a phasedifference between the polarized component which is parallel to theoptical axis of the transmitted light and the polarized component whichis perpendicular to the optical axis when the second voltage is applied.

The Q switch may function as a quarter-wave plate for light with awavelength of laser light when the first voltage is applied.

The photoacoustic signal processing unit may generate a photoacousticimage on the basis of a photoacoustic signal.

The laser light source may further have an interrupter closing detectorthat detects the closing of the optical path shutter. In this case, whenthe interrupter closing detector detects that the optical path shutteris closed, the voltage applied to the Q switch may be controlled to bethe second voltage.

When an instruction to measure a photoacoustic signal is input, thephotoacoustic measurement device may operate in the first operationmode. When an instruction to stop the measurement of the photoacousticsignal is input, the photoacoustic measurement device may operate in thesecond operation mode.

According to the invention, there is provided a laser light sourceincluding: a laser medium; an excitation unit that excites the lasermedium; an optical resonator including a pair of mirrors that face eachother with the laser medium interposed therebetween; a Q switch that isprovided on an optical path of the optical resonator and changes opticalloss of the optical resonator, according to a voltage applied, such thatthe optical loss of the optical resonator when a first voltage isapplied to the Q switch is more than the optical loss of the opticalresonator when a second voltage lower than the first voltage is appliedto the Q switch; and an optical path shutter that is provided on anoptical path of laser emission light and switches the transmission andblocking of the laser emission light. In a first operation mode in whichlaser emission is performed, the optical path shutter transmits thelight from the laser light source and the voltage applied to the Qswitch is changed from the first voltage to the second voltage to emitpulsed laser light after the excitation unit excites the laser medium.In a second operation mode in which the laser emission is interruptedand waited for, the optical path shutter blocks the light from the laserlight source and the second voltage is applied to the Q switch.

In the photoacoustic measurement device according to the invention, whenthe first voltage, which is a high voltage, is applied, the Q switchinserted into the resonator increases the optical loss of the opticalresonator. When the second voltage, which is a low voltage, is applied,the Q switch decreases the optical loss of the optical resonator. Theoptical path shutter is provided on the optical path of laser emissionlight. In the second operation mode in which laser emission isinterrupted and the laser emission is waited for, the optical pathshutter is closed and the second voltage, which is a low voltage, isapplied to the Q switch. In the second operation mode, when theexcitation unit excites the laser medium, laser oscillation that isweaker than Q switch oscillation occurs in the optical resonator sincethe second voltage, which is a low voltage, is applied to the Q switch.However, since the optical path shutter is closed, oscillation light isprevented from being emitted prior to the optical path shutter.According to this structure, it is not necessary to continuously apply ahigh voltage to the Q switch in order to prevent the emission of lighteven when the waiting time for which no light is emitted is long and itis possible to suppress deterioration of the Q switch. In addition,since a separate quarter-wave plate does not need to be provided in theoptical resonator, it is possible to simplify the structure of theoptical resonator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a photoacoustic measurementdevice according to a first embodiment of the invention.

FIG. 2 is a block diagram illustrating the structure of a laser lightsource unit.

FIG. 3 is a timing chart illustrating the operation waveform of eachunit.

FIG. 4 is a block diagram illustrating a laser light source unit of aphotoacoustic measurement device according to a second embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the invention will be described in detailwith reference to the drawings. FIG. 1 illustrates a photoacousticmeasurement device according to a first embodiment of the invention. Aphotoacoustic measurement device 10 includes an ultrasonic probe (probe)11, an ultrasonic unit 12, and a laser light source unit 13. In theembodiment of the invention, ultrasonic waves are used as photoacousticwaves. However, the photoacoustic waves are not limited to theultrasonic waves. For example, acoustic waves with an audio frequencymay be used as long as an appropriate frequency can be selectedaccording to a subject or measurement conditions.

The laser light source unit 13 emits pulsed laser light to be radiatedto the subject. The laser light emitted from the laser light source unit13 is guided to the probe 11 by a light guide unit, such as an opticalfiber, and is then radiated from the probe 11 to the subject. Theirradiation position of the laser light is not particularly limited. Thelaser light may be radiated from a position other than the probe 11. Inthe subject, a light absorber absorbs the energy of the radiated laserlight and ultrasonic waves (photoacoustic waves) are generated.

The probe 11 includes an ultrasonic wave detector. The probe 11includes, for example, a plurality of ultrasonic detector elements(ultrasonic transducers) which are one-dimensionally arranged anddetects the photoacoustic waves emitted from the subject using theultrasonic transducers which are one-dimensionally arranged. Inaddition, the probe 11 may transmit ultrasonic waves to the subject anddetect reflected ultrasonic waves of the transmitted ultrasonic waves.

The ultrasonic unit 12 includes a receiving circuit 21, a photoacousticimage generation unit 22, and a control circuit 23. The receivingcircuit 21 receives the detected signal of the photoacoustic waves(photoacoustic signal) detected by the probe 11. The photoacoustic imagegeneration unit 22 is a signal processing unit and generates aphotoacoustic image on the basis of the received photoacoustic signal.The generation of the photoacoustic image includes, for example, thereconstruction, detection, and logarithmic conversion of thephotoacoustic signal. The ultrasonic unit 12 may further include anultrasonic image generation unit that generates an ultrasonic image onthe basis of the detected signal (reflected ultrasonic signal) of thereflected ultrasonic waves detected by the probe 11.

The generation of the image by the ultrasonic unit 12 is notindispensable. The photoacoustic image generation unit 22 may performany type of signal processing for the photoacoustic signal. In addition,the ultrasonic image generation unit may perform any type of signalprocessing for the reflected ultrasonic signal.

The control circuit 23 is a control unit and controls each component ofthe ultrasonic unit 12. In addition, the control circuit 23 transmits acontrol signal to the laser light source unit 13 or controls, forexample, laser emission. Specifically, the control circuit 23 outputs aflash lamp trigger signal, a Q switch trigger signal, and an opticalpath interruption signal to the laser light source unit 13.

FIG. 2 illustrates the structure of the laser light source unit 13. Thelaser light source unit 13 includes a laser rod 31, a flash lamp 32,mirrors 33 and 34, a Q switch 35, a flash lamp power supply unit 37, a Qswitch driver 38, and an optical path shutter 39. The laser rod 31 is alaser medium. For example, an alexandrite crystal can be used as thelaser rod 31. The flash lamp 32 is an excitation unit and emitsexcitation light to the laser rod 31. The laser rod 31 and the flashlamp 32 are accommodated in the excitation chamber 36. For example, theexcitation chamber 36 is provided with a cooling device and theexcitation chamber 36 is maintained at a constant temperature.

The mirrors 33 and 34 face each other with the laser rod 31 interposedtherebetween. An optical resonator is formed by the mirrors 33 and 34.It is assumed that the mirror 34 is arranged on the output side. The Qswitch 35 is inserted into the optical resonator. The Q switch 35includes, for example, a Pockels cell. For example, Impact10manufactured by Gooch & Housego PLC or Q1059 manufactured by Fast PulseTechnology, Inc. can be used as the Q switch 35. The Q switch 35 changesthe optical loss of the optical resonator depending on the voltageapplied. The Q switch 35 rapidly changes the insertion loss of theoptical resonator from a large value (low Q) to a small value (high Q)to obtain pulsed laser light.

The Q switch 35 increases the insertion loss of the optical resonatorwhen a high voltage is applied and decreases the insertion loss of theoptical resonator when no voltage is applied. In other words, theoptical loss of the optical resonator when a first voltage is applied tothe Q switch is more than the optical loss of the optical resonator whena second voltage lower than the first voltage is applied to the Qswitch. The first voltage is, for example, about 3 kV and the secondvoltage is, for example, 0 V (no voltage).

When the first voltage is applied, the Q switch (Pockels cell) 35 causesa predetermined phase difference between a polarized component which isparallel to the optical axis of transmitted light and a polarizedcomponent which is perpendicular to the optical axis. The predeterminedphase difference is, for example, π/2. In this case, the Q switch 35functions as a quarter-wave plate for light having the wavelength oflaser light. When the second voltage is applied, the Q switch 35 doesnot cause a phase difference between the polarized component which isparallel to the optical axis of the transmitted light and the polarizedcomponent which is perpendicular to the optical axis. That is, the Qswitch 35 transmits light, without changing the polarized state. Thecase in which the first voltage is applied corresponds to the turn-offof the Q switch and the case in which the second voltage is appliedcorresponds to the turn-on of the Q switch.

The Q switch 35 preferably increases the optical loss of the opticalresonator to the extent that laser oscillation does not occur when thefirst voltage is applied. The invention is not limited to the structurein which, when the first voltage is applied, the Q switch 35 functionsas the quarter-wave plate. The Q switch 35 preferably decreases theoptical loss of the optical resonator to the extent that laseroscillation occurs when the second voltage is applied. The invention isnot limited to the structure in which, when the second voltage isapplied, the Q switch 35 transmits light, without changing the polarizedstate. It is preferable that the Q switch 35 functions as thequarter-wave plate when the first voltage is applied and transmitslight, without changing the polarized state, when the second voltage isapplied, in order to obtain pulsed laser light with high power and shortpulse duration.

The flash lamp power supply unit 37 drives the flash lamp 32. The flashlamp power supply unit 37 applies a voltage between the electrodes ofthe flash lamp. When receiving the flash lamp trigger signal from thecontrol circuit 23, the flash lamp power supply unit 37 applies avoltage of a few kilovolts to a trigger electrode of the flash lamp 32.When a high voltage is applied to the trigger electrode, the flash lamp32 emits light.

The Q switch driver 38 applies a high-voltage pulse to the Q switch 35to control the turn-on and turn-off of the Q switch. The turn-on andturn-off of the Q switch are controlled by the Q switch trigger signalfrom the control circuit 23.

The optical path shutter 39 is arranged on the optical path of thepulsed laser light emitted from the output-side mirror 34. The opticalpath shutter 39 is controlled to switch between a closed state in whichit blocks the pulsed laser light such that the pulsed laser light is notemitted to the subject and an open state in which it transmits thepulsed laser light such that the pulsed laser light is emitted to thesubject. The opening and closing of the optical path shutter 39 arecontrolled by the optical path interruption signal from the controlcircuit 23. The optical path shutter 39 may be a mechanical shutter or acombination of a polarizer and an electro-optical element.

Here, the operation mode of the laser light source unit 13(photoacoustic measurement device 10) includes a first operation mode inwhich laser light is emitted and a second operation mode in which laseremission is interrupted and the laser emission is waited for. Thecontrol circuit 23 opens the optical path shutter 39 in the firstoperation mode. The control circuit 23 transmits the flash lamp triggersignal to the flash lamp power supply unit 37 to turn on the flash lamp32. The Q switch driver 38 applies a voltage of about 3 kV to the Qswitch 35 to turn off the Q switch 35 before the flash lamp 32 is turnedon. After the flash lamp 32 is turned on and the laser rod 31 issufficiently excited, the control circuit 23 transmits the Q switchtrigger signal to the Q switch driver 38. The Q switch driver 38temporarily reduces the voltage applied to the Q switch 35 to 0 V tochange the Q switch 35 from an off state to an on state, in response tothe Q switch trigger signal. The insertion loss of the optical resonatoris switched from a large value to a small value to generate laseroscillation. Then, pulsed laser light is emitted from the output-sidemirror 34.

In the second operation mode, the control circuit 23 closes the opticalpath shutter 39, using the optical path interruption signal. Inaddition, the control circuit 23 controls the voltage applied from the Qswitch driver 38 to the Q switch 35 such that the voltage is reduced to0 V, using the Q switch trigger signal. Since the voltage applied to theQ switch 35 is maintained at 0 V, the Q switch 35 is maintained in theon state. In this case, the control circuit 23 periodically transmitsthe flash lamp trigger signal to the flash lamp power supply unit 37 tomaintain the on state of the flash lamp 32. When the flash lamp 32 isturned on, laser oscillation occurs in the optical resonator since the Qswitch 35 is in the on state while the flash lamp 32 is turned on.Therefore, laser light with a long pulse which has a longer pulseduration than that when Q switch oscillation is performed is emitted.The laser light with a long pulse is blocked by the optical path shutter39 and is not emitted to the subject.

FIG. 3 illustrates the operation waveform of each unit. For example,when the doctor operates a console to input an instruction to generate aphotoacoustic image (to measure a photoacoustic signal), the laser lightsource unit 13 operates in the first operation mode. Before the flashlamp 32 emits light, a voltage of about 3 kV is stored in a capacitor ofa trigger circuit included in the flash lamp power supply unit 37 (a).In addition, a high voltage of about 3 kV is applied to the Q switch 35and the Q switch is turned off (c). The optical path shutter 39 ismaintained in the opened state (d).

The control circuit 23 outputs the flash lamp trigger signal to theflash lamp power supply unit 37. When receiving the flash lamp triggersignal from the control circuit 23, the flash lamp power supply unit 37supplies the voltage from the capacitor to the trigger electrode of theflash lamp 32. When a high voltage is applied to the trigger electrode,discharge occurs in the flash lamp 32 and the flash lamp 32instantaneously emits light (b). After the flash lamp 32 emits light,charging to the capacitor of the trigger circuit starts and thecapacitor is charged to a voltage of about 3 kV again.

After outputting the flash lamp trigger signal, for example, after a fewhundreds of microseconds from the output of the flash lamp triggersignal, the control circuit 23 outputs the Q switch trigger signal tothe Q switch driver 38 (c). The Q switch driver 38 reduces the voltageapplied to the Q switch 35 to 0 V for a predetermined period of time toturn on the Q switch 35, in response to the Q switch trigger signal.When the Q switch 35 is changed from the off state to the on state,energy stored in the optical resonator oscillates at a time and steeppulsed laser light with a pulse duration of about a few nanoseconds to100 ns is emitted (e). After the pulsed laser light is emitted, the Qswitch driver 38 returns the voltage applied to the Q switch 35 to 3 kV(c) to return the Q switch 35 to the off state.

For example, when the doctor operates the console to input aninstruction to stop the generation of the photoacoustic image (themeasurement of the photoacoustic signal), the operation mode of thelaser light source unit 13 is changed from the first operation mode tothe second operation mode. When the operation mode is changed from thefirst operation mode to the second operation mode, the control circuit23 closes the optical path shutter 39 using the optical pathinterruption signal (d). In addition, the control circuit 23 switchesthe voltage applied from the Q switch driver 38 to the Q switch to 0 V,using the Q switch trigger signal (c). In the first operation mode, thevoltage applied to the Q switch 35 is maintained at 0 V.

The control circuit 23 outputs the flash lamp trigger signal to theflash lamp power supply unit 37. When receiving the flash lamp triggersignal from the control circuit 23, the flash lamp power supply unit 37supplies the voltage from the capacitor to the trigger electrode of theflash lamp 32. When a high voltage is applied to the trigger electrode,discharge occurs in the flash lamp 32 and the flash lamp 32instantaneously emits light (b). This operation is the same as that inthe first operation mode.

When the flash lamp 32 emits light, laser oscillation occurs since the Qswitch 35 is in the on state. However, in this case, the laseroscillation is weaker than the Q switch laser oscillation in the firstoperation mode since it occurs immediately after the laser medium isexcited. Laser light with a long pulse which has a longer pulse durationthan that in the Q switch laser oscillation is emitted from theoutput-side mirror 34. The laser light with a long pulse is blocked bythe optical path shutter 39 in the closed state and is not output to theoutside (e).

In this embodiment, the case in which the first voltage, which is a highvoltage, is applied to the Q switch corresponds to the turn-off of the Qswitch and the case in which the second voltage, which is a low voltage,is applied to the Q switch corresponds to the turn-on of the Q switch.According to this structure, it is not necessary to provide thequarter-wave plate which needs to be provided in the optical resonatorwhen the case in which a high voltage is applied to the Q switchcorresponds to the turn-on of the Q switch and the case in which a lowvoltage (0 V) is applied to the Q switch corresponds to the turn-off ofthe Q switch. Therefore, it is possible to simplify the internalstructure of the optical resonator.

In this embodiment, in the second operation mode in which laser emissionis not performed and the laser emission is waited for, the optical pathshutter 39 is in the closed state and the voltage applied to the Qswitch 35 is controlled to be the second voltage which is a low voltage.When the second voltage is applied, the Q switch 35 is turned on and theflash lamp 32 emits light. Then, in the optical resonator, laseroscillation occurs. However, the laser light is blocked by the opticalpath shutter 39 and is not emitted to the outside. Therefore, it is notnecessary to stop the excitation of the laser medium by the flash lamp32. In this embodiment, in the second operation mode, the voltageapplied to the Q switch is switched to the second voltage which is a lowvoltage. Therefore, in the standby mode, it is possible to prevent ahigh voltage from being continuously applied to the Q switch and toprevent deterioration of the Q switch.

Next, a second embodiment of the invention will be described. FIG. 4illustrates a laser light source unit of a photoacoustic measurementdevice according to the second embodiment of the invention. A laserlight source unit 13 a according to this embodiment includes aninterrupter closing detector 40 in addition to the structure of thelaser light source unit 13 according to the first embodiment illustratedin FIG. 2. The interrupter closing detector 40 detects the closing ofthe optical path shutter 39.

When the interrupter closing detector 40 detects that the optical pathshutter 39 is closed, the laser light source unit 13 a operates in thesecond operation mode. Specifically, for example, the control circuit 23closes the optical path shutter 39 using the optical path interruptionsignal. After the interrupter closing detector 40 detects that theoptical path shutter 39 is closed, the control circuit 23 changes thevoltage applied to the Q switch 35 to 0 V, using the Q switch triggersignal. According to this structure, before the optical path shutter 39is closed, the voltage applied to the Q switch 35 is changed to 0 V.Therefore, it is possible to prevent the flash lamp 32 from being turnedon and thus to prevent laser light with a long pulse from being emittedto the outside. As a result, it is possible to improve safety.

For example, when the doctor operates the console to input aninstruction to stop the generation of a photoacoustic image, the controlcircuit 23 closes the optical path shutter 39 in order to change theoperation mode to the second operation mode. After the closing of theoptical path shutter 39 is detected, the control circuit 23 controls thevoltage applied to the Q switch 35 to be 0 V. Alternatively, forexample, after the doctor operates the console to input an instructionto close the optical path shutter 39 and the closing of the optical pathshutter 39 is detected, the control circuit 23 may control the voltageapplied to the Q switch 35 to be 0 V, thereby changing the operationmode to the second operation mode.

In FIG. 2, the flash lamp 32 is used as the excitation unit. However,the excitation unit is not limited to the flash lamp 32. Light sourcesother than the flash lamp 32 may be used as the excitation light source.In FIG. 2, a wavelength selection unit for controlling an oscillationwavelength may be inserted into the optical resonator and the laserlight source unit 13 may be a variable-wavelength laser which switcheslight components with a plurality of wavelengths and emits the lightcomponent.

In the above-described embodiments, for example, when the doctor stopsthe generation of the photoacoustic image, the laser light source unit132 operates in the second operation mode. However, the invention is notlimited thereto. For example, the ultrasonic unit 12 may have threeoperation modes for generating images, that is, an operation mode forgenerating only an ultrasonic image, an operation mode for generatingonly a photoacoustic image, and an operation mode for generating both aphotoacoustic image and an ultrasonic image. When the ultrasonic unit 12operates in the operation mode in which no photoacoustic image isincluded in a generated image, the laser light source unit 13 mayoperate in the second operation mode. Alternatively, for example, whenthe probe 11 or the console of the ultrasonic unit 12 is not operatedfor a predetermined period of time or more, the laser light source unit13 may operate in the second operation mode.

The preferred embodiments of the invention have been described above.However, the photoacoustic measurement device and the laser light sourceaccording to the invention are not limited only to the above-describedembodiments and various modifications and changes of the structuresaccording to the above-described embodiments are also included in thescope of the invention.

What is claimed is:
 1. A photoacoustic measurement device comprising: alaser medium; an excitation unit that excites the laser medium; anoptical resonator including a pair of mirrors that face each other withthe laser medium interposed therebetween; a Q switch that is provided onan optical path of the optical resonator and changes optical loss of theoptical resonator, according to a voltage applied, such that the opticalloss of the optical resonator when a first voltage is applied to the Qswitch is more than the optical loss of the optical resonator when asecond voltage lower than the first voltage is applied to the Q switch;a laser light source that is provided on an optical path of laseremission light and has an optical path shutter which switches thetransmission and blocking of the laser emission light; an acoustic wavedetection unit that, after light is emitted from the laser light sourceto a subject, detects a photoacoustic wave generated by the emission ofthe light; a photoacoustic signal processing unit that performs signalprocessing for the photoacoustic wave; and a control unit that performscontrol such that, in a first operation mode in which laser emission isperformed, the optical path shutter transmits the light from the laserlight source and the voltage applied to the Q switch is changed from thefirst voltage to the second voltage to emit pulsed laser light after theexcitation unit excites the laser medium; and such that, in a secondoperation mode in which the laser emission is interrupted and waitedfor, the optical path shutter blocks the light from the laser lightsource and the second voltage is applied to the Q switch, wherein thelaser light source has an interrupter closing detector that detects theclosing of the optical path shutter, and when the interrupter closingdetector detects that the optical path shutter is closed, the controlunit controls the voltage applied to the Q switch to be the secondvoltage.
 2. The photoacoustic measurement device according to claim 1,wherein, in the second operation mode, the excitation unit periodicallyexcites the laser medium.
 3. The photoacoustic measurement deviceaccording to claim 2, wherein the second voltage is 0 V.
 4. Thephotoacoustic measurement device according to claim 3, wherein the Qswitch gives a predetermined phase difference between a polarizedcomponent which is parallel to an optical axis of transmitted light anda polarized component which is perpendicular to the optical axis whenthe first voltage is applied and does not give a phase differencebetween the polarized component which is parallel to the optical axis ofthe transmitted light and the polarized component which is perpendicularto the optical axis when the second voltage is applied.
 5. Thephotoacoustic measurement device according to claim 3, wherein the Qswitch functions as a quarter-wave plate for light with a wavelength oflaser light when the first voltage is applied.
 6. The photoacousticmeasurement device according to claim 4, wherein the Q switch functionsas a quarter-wave plate for light with a wavelength of laser light whenthe first voltage is applied.
 7. The photoacoustic measurement deviceaccording to claim 2, wherein the Q switch gives a predetermined phasedifference between a polarized component which is parallel to an opticalaxis of transmitted light and a polarized component which isperpendicular to the optical axis when the first voltage is applied anddoes not give a phase difference between the polarized component whichis parallel to the optical axis of the transmitted light and thepolarized component which is perpendicular to the optical axis when thesecond voltage is applied.
 8. The photoacoustic measurement deviceaccording to claim 7, wherein the Q switch functions as a quarter-waveplate for light with a wavelength of laser light when the first voltageis applied.
 9. The photoacoustic measurement device according to claim2, wherein the Q switch functions as a quarter-wave plate for light witha wavelength of laser light when the first voltage is applied.
 10. Thephotoacoustic measurement device according to claim 1, wherein thesecond voltage is 0 V.
 11. The photoacoustic measurement deviceaccording to claim 10, wherein the Q switch gives a predetermined phasedifference between a polarized component which is parallel to an opticalaxis of transmitted light and a polarized component which isperpendicular to the optical axis when the first voltage is applied anddoes not give a phase difference between the polarized component whichis parallel to the optical axis of the transmitted light and thepolarized component which is perpendicular to the optical axis when thesecond voltage is applied.
 12. The photoacoustic measurement deviceaccording to claim 11, wherein the Q switch functions as a quarter-waveplate for light with a wavelength of laser light when the first voltageis applied.
 13. The photoacoustic measurement device according to claim10, wherein the Q switch functions as a quarter-wave plate for lightwith a wavelength of laser light when the first voltage is applied. 14.The photoacoustic measurement device according to claim 1, wherein the Qswitch gives a predetermined phase difference between a polarizedcomponent which is parallel to an optical axis of transmitted light anda polarized component which is perpendicular to the optical axis whenthe first voltage is applied and does not give a phase differencebetween the polarized component which is parallel to the optical axis ofthe transmitted light and the polarized component which is perpendicularto the optical axis when the second voltage is applied.
 15. Thephotoacoustic measurement device according to claim 14, wherein the Qswitch functions as a quarter-wave plate for light with a wavelength oflaser light when the first voltage is applied.
 16. The photoacousticmeasurement device according to claim 1, wherein the Q switch functionsas a quarter-wave plate for light with a wavelength of laser light whenthe first voltage is applied.
 17. The photoacoustic measurement deviceaccording to claim 1, wherein the photoacoustic signal processing unitgenerates a photoacoustic image on the basis of the photoacousticsignal.
 18. The photoacoustic measurement device according to claim 1,wherein the photoacoustic measurement device operates in the firstoperation mode when an instruction to measure the photoacoustic signalis input and operates in the second operation mode when an instructionto stop the measurement of the photoacoustic signal is input.
 19. Thephotoacoustic measurement device according to claim 1, wherein, in thefirst operation mode, after changing the voltage applied to the Q switchfrom the first voltage to the second voltage, the control unit performscontrol such that the first voltage is applied to the Q switch.
 20. Alaser light source used in the photoacoustic measurement deviceaccording to claim 1, comprising: a laser medium; an excitation unitthat excites the laser medium; an optical resonator including a pair ofmirrors that face each other with the laser medium interposedtherebetween; a Q switch that is provided on an optical path of theoptical resonator and changes optical loss of the optical resonator,according to a voltage applied, such that the optical loss of theoptical resonator when a first voltage is applied to the Q switch ismore than the optical loss of the optical resonator when a secondvoltage lower than the first voltage is applied to the Q switch; and anoptical path shutter that is provided on an optical path of laseremission light and switches the transmission and blocking of the laseremission light, wherein, in a first operation mode in which laseremission is performed, the optical path shutter transmits the light fromthe laser light source and the voltage applied to the Q switch ischanged from the first voltage to the second voltage to emit pulsedlaser light after the excitation unit excites the laser medium, and in asecond operation mode in which the laser emission is interrupted andwaited for, the optical path shutter blocks the light from the laserlight source and the second voltage is applied to the Q switch.